1. Field of the Invention
The present invention relates to a color picture tube and, more particularly, to a supporting assembly for connecting a shadow mask to the front panel of a color picture tube.
2. Description of the Related Art
A color picture tube comprises three electron guns, a front panel, a phosphor screen formed on the inner surface of the front panel, a rigid mask frame, and a shadow mask supported by the rigid mask frame and facing the phosphor screen. The shadow mask has a plurality of apertures through which electron beams emitted from the guns pass before striking the phosphor screen. The rigid mask frame surrounds the peripheral edge of the shadow mask, thus maintaining the shape of the shadow mask. The color picture tube further comprises a supporting assembly which supports the mask frame. The mask frame can be detached from the supporting assembly, as can the shadow mask.
As a result of the electron beam passing through the apertures of the shadow mask for some time, the shadow mask becomes heated and expands, which causes the positions of the individual apertures to change. This inevitably gives rise to an undesirable phenomenon known as "doming", i.e., the mislanding of the beams. To compensate for the thermal expansion of the mask, a bimetal is incorporated in the supporting assembly. This bimetal functions to move the shadow mask as the mask thermally expands, so that the electron beams passing through the apertures of the mask can land on the desired portions of the phosphor screen. However, the bimetal cannot move the shadow mask sufficiently quickly after the mask has begun to thermally expanded, and thus mislanding of the beam occurs. Besides, incorporating the bimetal, which is relatively heavy, renders the picture tube undesirably heavy.
Japanese patent Publication No. 46-4104 discloses a rectangular mask frame having four leaf springs which project from each of the four corners and are connected to the four corners of a shadow mask, thereby supporting the shadow mask. As a result, when the shadow mask thermally expands, deformation of the mask frame is less than that of the mask frame surrounding the shadow mask. The use of this mask frame results in the following advantages:
(1) Mislanding of the beams, does not occur in concentration at a particular portion of the phosphor screen. PA1 (2) Mislanding of the beam, due to the vibration of the mask frame, is reduced since the springs absorb the vibration. PA1 (3) The weight of the picture tube can be reduced significantly since the frame is thin and lightweight.
FIG. 1 shows a proposed assembly for supporting the shadow mask of a color picture tube, which is a combination made up of a stud pin 24 which projects from an inner peripheral wall of panel 11, and a leaf spring 2 having a hole through which stud pin 24 extends. Stud pin 24 is tapered at an angle of about 12.degree.. Leaf spring 2 is composed of three portions, i.e., fixed portion 3, welded to frame 18 and located near phosphor screen 14; elastically deformable portion 4; and engaging portion 5, supported by stud pin 24, on its rear side, and located remote from screen 14. Leaf spring 2 has a substantially V-shaped longitudinal section. Ideally, the angle (.theta.) defined by movable portion 4 and frame 18 should be 45.degree. in the case of a picture tube having a beam deflection angle of 90.degree., and 35.degree. in the case of a picture tube having a beam deflection angle of 110.degree., in order to compensate only for the mislanding of electron beams 15 which has resulted from the thermal expansion of shadow mask 17.
After the picture tube of either of the above types has been in continuous use for 30 minutes or more, frame 18 begins to expand due to beam heating. Since the diameter of frame 18 inevitably increases as a result of thermal expansion, allowing shadow mask 17 to move, as is shown in FIG. 1, apertures 19 of shadow mask 17 (indicated by a solid line) move in the radial direction of the picture tube. Nonetheless, leaf springs, which are inclined to shadow mask 17, push frame 18 toward phosphor screen 14, thus moving shadow mask 17 toward screen 14, whereby apertures 19 move to position 19b. Therefore, mislanding of the beams, resulting from the expansion of mask 17, is reduced to some extent, with a corresponding decrease in what is known as color purity drift.
However, in a conventional supporting assembly, when frame 18 is displaced in the radial direction, as indicated by an alternate long and two short dashed line in FIG. 2, engaging portion 5 of spring 2 is inclined, and portion 4 is curved. For this reason, the shift amount of frame 18 in the axial direction becomes smaller than a design value. Thus, the mask assembly cannot be moved for a sufficiently long distance in the axial direction when spring 2 is attached to frame 18 at the above-mentioned angle. In this case, apertures 19 of mask 17 cannot be located at the desired positions.
Spring 2 is attached to frame 18 to have an angle .theta. larger than the above-mentioned value, e.g., to be about 70 degrees or more in a picture tube having a beam deflection angle of 90 degrees, so as to increase the shift amount of frame 18 by spring 2. However, if the angle .theta. is increased, the spring constant of spring 2 becomes too large, and it is difficult to detach shadow mask assembly from panel 11, resulting in a variety of problems in respect of manufacture and use.
More specifically, in manufacturing processes of a color picture tube, an attachment/detachment of a shadow mask to/from a panel must be repetitively performed. When leaf springs are used, the attachment/detachment cannot be manually performed and must be performed by a special-purpose apparatus installed for each process. Furthermore, a large force acts on the supporting assembly, the mask frame, and the shadow mask during attachment/detachment of the shadow mask, and may deform these parts.
When such strong springs are used, spring 2 is fatigued upon several attachment/detachment operations, and an initial spring constant cannot be maintained. Therefore, it is difficult to precisely maintain the shadow mask assembly by means of the supporting assembly without the shadow mask assembly drifting in the panel.
The thermal expansion coefficient of a material constituting a stud pin must be substantially the same as that constituting the panel, in order to prevent the stud pin from cracking at the projecting portion. For this reason, as a material of the stud pin, an Fe-Cr alloy (Cr: 18 wt. %) is normally employed. This material is soft, i.e., has a Vickers hardness (Hv) of about 150. Leaf spring 2 is normally formed of hard stainless steel having an Hv of 380 to 500. When a rigid spring is used, the stud pin is recessed at a contacting portion, and mounting precision of the shadow mask is degraded.
Therefore, in the conventional supporting assembly, the mounting angle .theta. cannot be increased, and purity drift in a continuous operation for a long period of time cannot be satisfactorily compensated. Therefore, a deterioration of color purity near a peripheral region of the screen occurs.